1. The Field of the Invention
The present invention relates to a method and apparatus for deploying and recovering marine seismic cables and in particular to a method and apparatus which will substantially reduce cable tension in both hauling and reeling the cables thereby substantially eliminating the risk of cable damage.
2. The Prior Art
It has long been known that there are substantial petroleum deposits located beneath the sea and that it is necessary to determine the exact location of these deposits in order to commence recovery operations. Marine seismic exploration has become an accepted and standardized practice using a number of different approaches. The most popular seismic exploration method utilizes an explosive source and at least one cable of substantial length equipped with a plurality of spaced hydrophones, and other supporting transducers and electronics, adapted to detect reflected energy from the sea's subbottom's geologic anomalies. The cables are generally referred to as streamers and range in length from one to four miles with from one to four cables or streamers being pulled behind a single survey vessel.
Each cable or streamer is formed by a number of segments or elements, each a water-tight tube about 3 inches in diameter and approximately 150 feet in length. The outer cover or jacket can be a single extrusion or a builtup laminate including reinforcement materials. Each cable segment contains an array of electronic devices including hydrophones and/or transducers and rigid cylindrical protective cans in a spaced configuration with the residue of the interior of the segment being filled with a special oil for flotation purposes. Each segment is connected to the adjacent segment by a rigid cylindrical can which is approximately 4 1/2 inches in diameter and 18 inches long. The total streamer will be an assembly of the above mentioned segments having an overall one to four mile length.
Each streamer is buoyant in that it will float unaided. Each streamer, while being rugged enough to withstand an ocean environment, is somewhat fragile in that it will not tolerate high line pulls or being wound under tension when stored. The previously mentioned cans and other internal blocks or spacers cause a particular problem when the cable is retrieved and/or stored in that, while the streamer is generally flexible, the blocks, spacers and cans are essentially rigid and therefore, will not tolerate being wound tightly either during recovery or in storage. The blocks, spacers and cans also cause a problem in that, if the streamer is wound too tightly, there is great danger of perforation of the outer cover by the sharp edges formed by the ends of these spacers, blocks and cans.
Normally, the cables are towed or streamed at about 5 mph, known as "shooting speed." This creates about 2500 ftlbs of tension on a 2 mile cable. Recovery of the cable necessarily increases tension on the cable and raises the possibility of damage. If only a single cable is deployed, then it is possible to back the vessel during the recovery operation thereby reducing tension. However, such a backing of the vessel is not possible when multiple cables have been deployed.